In the art of telecommunication, it is known to transfer signals, with or without amplification, between two line pairs. For example, a typical voice frequency repeater used in telephone networks is described in Freeman, Telecommunication Transmission Handbook, John Wiley & Sons, New York 1975 at Section 2.10. The repeater, shown in FIG. 2.13 therein, utilizes two unidirectional amplifiers in a four line circuit which, through the use of a pair of hybrid transformers, is connected via two lines to each of two stations.
Other illustrations of bidirectional amplifiers are found in the patent art. For example, U.S. Pat. No. 4,282,407 to Stiefel discloses a telephone loop resistance detector which utilizes a bidirectional amplifier 22. The bidirectional amplifier, shown in FIG. 2, includes a pair of unidirectional differential amplifiers 50 and 51 interconnected in a feedback arrangement for passing signals from tip and ring lines 52 and 54 to and from tip and ring lines 53 and 54, ring line 54 being common to the two line pairs. Positive feedback circuits 59, 60 and 62, 63 are used to subtract each signal from a composite signal. However, the impedance used in the feedback network is frequency sensitive, thus suggesting that the amount of feedback varies with frequency. No provision is made for complete elimination of receiver-transmitter feedback. The disclosed circuit provides a switchable gain.
A further illustration of a bidirectional amplifier is provided in U.S. Pat. No. 3,519,765 to Huber, wherein a single transistor 14 is used to amplify signals in both directions between two signal sending and receiving devices. A control switch 12 is used to reverse the bias voltage applied to the transistor in order to operate the transistor 14 in one or the other of the two modes of amplification. The transistor is constantly connected between both sending and receiving devices, thus permitting feedback between the transmitted and received signals.
Still another reference of interest is the Chambers, Jr. U.S. Pat. No. 3,573,402 wherein a bidirectional amplifier additively combines a power source with a signal source in one of two directions, depending on the direction of current in the signal loop. A controlled amount of boost voltage is added in a power aiding relationship to an input signal source, without provision for elimination of feedback between the two signal sources. The disclosed circuit requires sensing circuitry to determine the direction of flow of signal current in order to switch the polarity of connection of the power source to the circuitry. A circuit responsive to net current in the source-load loop is utilized towards that end.
In each of these prior art circuits there is provided a direct connection between the two line pairs and thus between the paths provided for the signals being transmitted bidirectionally through the disclosed circuitry. Thus, prior art circuitry typically leads to generation of feedback signals since a signal generated at a transmitter of a first station and placed on the lines is also provided to the receiver of the first station, as well as to the destination second station. Thus, where it is desired to add to, or extract from a signal, passing through a transmission line such as a telephone line by means of a connection to a microphone and speaker, for example, and where a bidirectional amplifier of the type disclosed in the prior art is used to connect the telephone line and loudspeaker, ringing and feedback will develop when the microphone and speaker are in close proximity.
There is thus a need in the prior art to provide apparatus and method for bidirectionally connecting a signal source to a receiver and transmitter station without generation of any ringing or feedback due to the connection itself.